Capacitive sensor interference determination

ABSTRACT

A processing system for a capacitive input device is described. The capacitive input device includes a plurality of sensor electrodes configured to detect input objects in a sensing region. The processing system configured to transmit a signal on a transmitter sensor channel of the capacitive input device. The processing system is also configured to receive the signal on a receiver sensor channel of the capacitive input device, wherein the receiver sensor channel is coupled with an amplifier. The processing system is also configured to determine if a level of interference has been received by the receiver sensor channel in conjunction with receipt of the signal.

CROSS-REFERENCE TO RELATED APPLICATIONS (CONTINUATION)

This application claims priority and is a continuation to the co-pendingpatent application Ser. No. 12/491,102, entitled “Capacitive SensorInterference Determination,” with filing date Jun. 24, 2009, andassigned to the assignee of the present application, the disclosure ofwhich is hereby incorporated herein by reference.

This application is related to U.S. patent application Ser. No.13/024,539 entitled INPUT DEVICE INTERFERENCE DETERMINATION, by VivekPant et al., assigned to the assignee of the present invention, filedFeb. 10, 2011.

BACKGROUND

Capacitive sensor devices, otherwise known as touch sensor devices orproximity sensors are widely used in modern electronic devices. Acapacitive sensor device is often used for touch based navigation,selection, or other input, in response to a finger, stylus, or otherobject being placed on or in proximity to a sensor of the capacitivesensor device. In such a capacity, capacitive sensor devices are oftenemployed in computers (e.g. notebook/laptop computers), media players,multi-media devices, remote controls, personal digital assistants, smartdevices, telephones, and the like.

Such capacitive sensor devices are often operated, at least in part, bya controller component such as an application specific integratedcircuit (ASIC). The inputs and/or outputs of the controller componentare typically used to drive the portions of the sensor devices and tomeasure capacitance(s) from the sensor devices. The measurements mayinclude multiple inputs and/or outputs (e.g. receivers, transmitters andguards, etc.) and can include absolute and transcapacitive measurements.

With respect to transcapacitance, some capacitive implementationsutilize transcapacitive sensing methods based on the capacitive couplingbetween sensor conductors. Transcapacitive sensing methods are sometimesalso referred to as “mutual capacitance sensing methods.” Atranscapacitive sensing method operates, for example, by detecting theelectric field coupling one or more transmitting sensor conductors withone or more receiving sensor conductors in a sensor array. Proximateobjects may cause changes in the electric field, and produce detectablechanges in the transcapacitive coupling. Sensor conductors may transmitas well as receive, either simultaneously or in a time multiplexedmanner. Sensor conductors that transmit are sometimes referred to as the“transmitting sensor electrodes,” “driving sensor electrodes,” “rowdrivers,” “transmitters,” or “drivers”—at least for the duration whenthey are transmitting. Other names may also be used, includingcontractions or combinations of the earlier names (e.g. “drivingelectrodes” and “driver electrodes.” Sensor conductors that receive aresometimes referred to as “receiving sensor electrodes,” “receiverelectrodes,” “column receivers,” or “receivers”—at least for theduration when they are receiving. Similarly, other names may also beused, including contractions or combinations of the earlier names. Inone implementation, a transmitting sensor electrode is modulatedrelative to a system ground to facilitate transmission. In anotherimplementation, a receiving sensor electrode is not modulated relativeto system ground to facilitate receipt.

With respect to a controller, the inputs/outputs of the controller maybe configured at different times to measure different capacitances fromthe sensor device. For example, a grid of capacitive sensor elements ofa capacitive sensor device's sensor array can be scanned to determineindividual transcapacitances that can be integrated to capacitivelyimage an input object or objects that are touching or proximate to thecapacitive sensor array of the capacitive sensor device.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe Description of Embodiments, illustrate various embodiments of thepresent invention and, together with the Description of Embodiments,serve to explain principles discussed below. The drawings referred to inthis Brief Description of Drawings should not be understood as beingdrawn to scale unless specifically noted.

FIG. 1 is a block diagram of an example capacitive sensor device,according to an embodiment.

FIG. 2 is a block diagram of an example receiver amplifier of acapacitive sensor device, according to an embodiment.

FIG. 3 is an example voltage and timing diagram for a receiveramplifier, according to an embodiment.

FIG. 4 is a block diagram of an example capacitive sensor interferencedetermining circuit implemented with a receiver amplifier of acapacitive sensor device, according to an embodiment.

FIG. 5 is a flow diagram a method of determining interference in acapacitance sensor device, according to an embodiment.

FIG. 6 is a flow diagram of a method of fabricating an interferencedetermining processor system for a capacitive sensor device, accordingto an embodiment.

DESCRIPTION OF EMBODIMENTS

Reference will now be made in detail to various embodiments of thesubject matter, examples of which are illustrated in the accompanyingdrawings. While various embodiments are discussed herein, it will beunderstood that they are not intended to limit to these embodiments. Onthe contrary, the presented embodiments are intended to coveralternatives, modifications and equivalents, which may be includedwithin the spirit and scope the various embodiments as defined by theappended claims. Furthermore, in this Description of Embodiments,numerous specific details are set forth in order to provide a thoroughunderstanding of embodiments of the present subject matter. However,embodiments may be practiced without these specific details. In otherinstances, well known methods, procedures, components, and circuits havenot been described in detail as not to unnecessarily obscure aspects ofthe described embodiments.

Notation and Nomenclature

Unless specifically stated otherwise as apparent from the followingdiscussions, it is appreciated that throughout the present Descriptionof Embodiments, discussions utilizing terms such as “transmitting,”“receiving”, “determining,” “using,” “comparing” or the like, oftenrefer to the actions and processes of discrete electrical components(e.g., amplifiers, capacitors, resistors, and the like) or anarrangement of components in an integrated circuit (e.g., an applicationspecific integrated circuit (ASIC)) or similar electronic device. Theelectronic device transmits, receives, manipulates and/or transformssignals represented as physical (electrical) quantities within theelectronic device's circuits, components, logic, and the like, intoother signals similarly represented as physical electrical quantitieswithin the electronic device or within or transmitted to otherelectronic or computing devices.

Overview of Discussion

Capacitive sensor devices are susceptible to interference that isreceived along with a sensed capacitance. Interference can cause sampledcapacitances that are measured from a sensor array to be inaccurateand/or unusable when an amplifier coupled with a receiver output of thesensor array is forced into a non-linear range of amplification due tothe presence of interference in a received signal that is beingamplified. Conventionally, this can be problematic when measuringcapacitances from a sensor array, as a sampled capacitance value may beinvalid due to corruption by excess interference which can eithersaturate the receiver amplifier or prevent it from settling to a levelat which an accurate measurement can be made.

Herein, methods and circuits are described that can be used to detectthe presence of interference in a capacitance measuring system. Whenemployed in a controller for a capacitive sensor device, these methodsand circuits operate to detect corrupted measurement samples right atthe time that the corruption due to interference occurs. In this manner,an indication of an interference corrupted measurement, due to eitherinsufficient settling or saturation, is generated contemporaneously withthe sampling period for the measurement. This alerts the controller andpresents the opportunity to take any one of a number of actions withrespect to the interference corrupted measurement sample.

Discussion will begin with a description of an example capacitive sensordevice. This discussion will include description of example inputsignals and output signals. Operation of the receiver amplifier will bedescribed in conjunction with description of an example timing diagram.A block diagram of an example capacitive sensor interference determiningcircuit used in conjunction with a receiver amplifier of a capacitivesensor device will be presented. Discussion will then be directed towardan example method of determining interference in a capacitance sensordevice, in accordance with the embodiments described herein. Finally, anexample method of fabricating an interference determining processorsystem for a capacitive sensor device, in accordance with theembodiments described herein, will be discussed.

Example Capacitive Sensor Device

FIG. 1 is a block diagram of an example capacitive sensor device 100,according to an embodiment. Capacitive sensor device 100 comprises acontroller 105 and a sensor array 110. In one embodiment, controller 105is implemented as an integrated circuit, such as application specificintegrated circuit (ASIC). Controller 105 comprises a transmit side anda receive side. The transmit side includes a plurality of drivers,illustrated as transmitters (XMTR1 to XMTRn), each of which isconfigured to generate and transmit a carrier signal on a row driver 111(e.g., an active sensor conductor row 111). The receive side ofcontroller 105 includes a plurality of receiver amplifiers, illustratedas receiver amplifiers (121-1 to 121-n), that amplify a received signalsfrom a column receiver 112 (e.g., an active sensor conductor column 112)of sensor array 110.

As illustrated, each receiver amplifier 121 includes a non-invertinginput coupled with a reference voltage (Vref). Each receiver amplifier121 also includes a feedback capacitor (Cfb) coupled between the outputand the inverting input of the receiver amplifier 121. Additionally,each receiver amplifier 121 is configured with a switch across Cfb thatcan be dosed to create a reset condition. A boxed detail area 120surrounds receiver amplifier 121-1 and its inputs and outputs. Detail120 is illustrated in greater specificity in FIGS. 2 and 4.

As shown in FIG. 1, sensor array 110 comprises a plurality of sensorconductors arranged in a matrix of rows 111 (111-1, 111-2, 111-3 . . .111-n) and columns 112 (112-1, 112-3, 112-3 . . . 112-n). As describedabove, when active, a row 111 is often referred to as a row driver and acolumn, when active, is often referred to as a column receiver. Althoughfour rows 111 and four columns 112 are illustrated, it is appreciatedthat in other embodiments a greater or lesser number of row 111 and/orcolumn 112 can be included in sensor array 110. Coupled with each row111 is a driver in the form of a transmitter, such as XMTR1, whichdrives a carrier signal into the row 111 to which it is coupled. Thiscarrier signal is capacitively coupled with the columns 112, such as atrow/column intersection 130-1, and then output (Rcvr_1 to Rcvr_n) on acolumn receiver 112 to be amplified by a receiver amplifier 121, that iscoupled with a particular column 112. Due to space constraints, only tworeceiver amplifiers 121 are illustrated in FIG. 1. However it isappreciated that controller 105 may be configured with a separatereceiver amplifier 121 coupled with each column 112 of sensor array 110.

In operation, on the transmit side a carrier is generated and driveninto the rows 111 of the matrix-arranged sensor array 110 bytransmitters XMTR1 through XMTRn. The presence of one or more objects,such as fingers, touching or in proximity to sensor array 110 adds tothe current already injected into the rows 111 of sensor array 110. Theadded current is integrated in multiple receiver amplifiers connected tothe rows 111 of sensor array 110. By utilizing a timed scanning patternsimultaneously on both the transmitters (XMTR1 though XMTRn) andreceiver amplifiers 121, a capacitive image scan of objects touching orin close proximity to sensor array 110 can be generated throughevaluation of a plurality of measured samples taken while driving sensorarray 110.

In operation of controller 105, according to one embodiment, on eachreceiver channel (Rcvr_1 through Rcvr_n), the output (i.e. Rcvr_Out_1)of the receiver amplifier 121 that is coupled with the receiver channelis further processed by controller 105. This further processing caninclude removing the carrier signal with a demodulator and ananti-aliasing low pass filter. This further processing can also includefiltering an input device signal (referred to herein as a fingersignal). The filtered finger signal is held by an analog to digitalconvertor to create a digital equivalent output to be used by firmwareof controller 105 or another electronic device. It is appreciated thatcircuitry for performing such filtering and analog to digital conversionis well known in the art and is not illustrated herein so as not toobscure other aspects of controller 105.

During operation of capacitive sensor device 100, interference or highamplitude external signals may be directly coupled into sensor array 110or coupled into sensor array 110 via an object such as a finger that isproximate to or touching sensor array 110. When excessive amounts ofsuch interference are coupled into the receiver channels this cancorrupt the signals output by receiver amplifiers 121. This corruptioncan occur through two different mechanisms. First, high amplitudeinterference can saturate (peg) the output of a receiver amplifier 121,thus obscuring and invalidating a signal measured with respect to atouching or proximate object, such as a finger. Second, interference mayforce a slewing effect on the output of receiver amplifier 121 (e.g.,Rcvr_Out_1) if the interference has sharp transitioning edges. In thiscase, the output of receiver amplifies 121 cannot settle to a finalvalue within the right time frame to be sampled. Thus, this lack ofsettling can also obscure and invalidate a signal measured with respectto a touching or proximate object, such as a finger.

As illustrated in FIG. 1, receiver amplifiers 121 are equipped with anadditional output, Non-linearity_Out, which is set when a receiveramplifier 121 (e.g., 121-1) experiences amplification non-linearitieseither due to saturation or due to a lack of settling during the time ofa sample. The Non-linearity_Out output of receiver amplifiers 121 alertscontroller 105 that the presence of excessive interference has beendetermined. This determination allows controller 105 to deal with thisinterference according to one or more predefined or dynamic responseswhich may be implemented in hardware or firmware of controller 105 oranother electronic device communicatively coupled with controller 105.By way of example and not of limitation, in various embodiments, a highoutput on Non-linearity_Out can trigger rejection of the Rcvr_Out signal(e.g., controller 105 can disregard or not sample the output of Rcvr_Outor can disallow a sample from Rcvr_Out to be filtered/averaged withprevious samples); controller 105 can count occurrences of interferencecontaminated/corrupted samples (e.g., the number of occurrences withinan analog to digital conversion cycle) and take a predetermined actionbased on the count; and/or hardware or firmware based algorithms ofcontroller 105 or another electronic device can normalize the value ofinterference corrupted samples (e.g., by averaging surroundinguncorrupted samples) before using them.

The operation of a receiver amplifier 121 and an example of theimplementation of the Non-linearity_Out output of a receiver amplifier121 is described in greater specificity in conjunction with descriptionof detail region 120 in FIGS. 2 and 4, the example timing diagram ofFIG. 3, and the example method of operation of FIG. 5.

FIG. 2 is a block diagram of detail 120, which shows an example receiveramplifier 121-1 of a capacitive sensor device, according to anembodiment. The block diagram of detail 120 also shows the inputs andoutputs to receiver amplifier 121-1 and is included to illustratecomponents of input signal, Rcvr_1, that are received from a row/columnintersection, such as row/column intersection 130-1, of sensor array110. It is appreciated that sensing provided by row/column intersection130-1 is not limited to the actual intersection of a row 111 and column112, but instead encompasses some vicinity surrounding the intersection.An example of this “vicinity” is illustrated two-dimensionally by thedashed circle 130-1 of FIG. 1. It is appreciated, however that thisvicinity also extends in a third dimension which is not illustrated bythe plan view of FIG. 1. Likewise, it is appreciated that row/columnintersection 130-1 is illustrated as only a single example of arow/column intersection 130 and that a sensor array, such as sensorarray 110, typically includes a plurality of row/column intersections130.

Drive signals (e.g. from XMTR1) are driven in the range of 0 v to Vdd.Thus, Rcvr_1 varies between 0 and Vdd due to the component attributableto the drive signal that has been capacitively coupled from a row 111 toa column 112 of sensor array 110. Due to this, when driven, Rcvr_1 willinclude a voltage (Vdrive) that varies between 0 and Vdd and willinclude a capacitance (Ct) that is attributable to the internalcapacitance of sensor array 110 (i.e., the capacitance between thetransmit row 111-1 and receive column 112-1 to which receiver amplifier121-1 is coupled). Additionally, some level of interference (which mayincrease due to being coupled through a finger) will also be coupledinto sensor array 110 and included in the signal of Rcvr_1. When anobject/objects, generically described herein as a finger, iscapacitively coupled with the sensor element (e.g., row/columnintersection 130-1 in the illustrated example) of sensor array 110 thatthat outputs the Rcvr_1 signal, Rcvr_1 will additionally be modulatedwith some amount of signal attributable to the finger and theenvironment, and Rcvr_1 will also include a capacitive component (Cf)that is attributed to the finger.

Receiver amplifier 121-1 is a transconductance amplifier. As such, thenominal Rcvr_Out output of receiver amplifier 121-1, absent interferenceor other input to sensor array 110, can be defined by either Equation 1or Equation 2, depending on which direction the drive signal (e.g.,XMTR1) is being driven:Rcvr_Out_(—)1=(−Ct/Cfb)Vdd+Vref  Equation 1Rcvr_Out_(—)1=(Ct/Cfb)Vdd+Vref  Equation 2

Consider an example where Vref=Vdd/2; Ct=2 pf; and Cfb=16 pf. Withreference to Equations 1 and 2, in such an example, the range of thesignal Rcvr_Out_1 can vary from a high of VDD/2+Ct/Cfb*VDD to a low ofVDD/2−Ct/Cfb*VDD. Thus, by substituting the example values for Ct andCfb, the equation for the high value becomes Vdd/2+Vdd/8=5/8 Vdd, andthe equation for the low value becomes Vdd/2−Vdd/8=3/8 Vdd. Thus,without any input such as a touch, the output, Rcvr_Out_1, of receiveramplifier 121-1 should exist within or very near this range.

For purposes of example, consider an embodiment where adding a fingercapacitance, Cf, changes the input capacitance to receiver amplifier121-1 by increasing it by another 2 pf. Thus the maximum value ofRcvr_Out_1 becomes Vdd/2+Vdd/4=3/4 Vdd and the minimum value ofRcvr_Out_1, becomes Vdd/2−Vdd/4=1/4 Vdd. When receiver amplifier 121-1is not slewing, the output will substantially settle in this range in apre-defined time period, such as, for example, 1 ms, after which time asample can be taken. Some amount of interference is typically present inthe Rcvr_Out_1 signal, however, when an erratic interference signal ispresent that causes slewing, receiver amplifier 121-1 will have aRcvr_Out_1 signal that is near the above described range, but is notsettled (i.e., it is oscillating or varying at an unacceptable levelthat prevents sampling of a reliable measurement). Additionally, whenreceiver amplifier 121-1 is saturated with interference, Rcvr_Out_1 willbe saturated to either Vdd or zero volts, both of which are well outsidethe expected range.

FIG. 3 is an example voltage and timing diagram for a receiveramplifier, such as receiver amplifier 121-1 of detail 120 (FIGS. 1 and2), according to an embodiment. For purposes of example, Vref will bedefined as Vdd/2, as was described above in conjunction with FIG. 2.Thus, following the example embodiment as described above in FIG. 2, theRcvr_Out_1 output varies from 0 to Vdd. FIG. 3 shows various possiblestates of Rcvr_Out_1, and it is appreciated that only one of the statesrepresented by signals 320, 325, 335, and 330 can occur at a time.Likewise, only one of the states represented by signals 355, 360, 365,and 370 can occur at a time.

Beginning from the left of FIG. 3, at location 310, a reset conditionhas brought Rcvr_Out_1 to Vdd/2. At 315, XMTR1 begins a positive driveon sensor array 110. Signal 320 shows the level of baseline static thatis present on the amplified signal Rcvr_Out_1, which is an example ofthe signal level of Rcvr_Out_1 without the presence of a finger or otherinput object. Although depicted as settled, it is appreciated that suchbaseline static can also be unsettled in some embodiments. Signal 325indicates an example settled level of Rcvr_Out_1 when a finger or otherinput object is present and sensed. Signal 325 is settled enough that itcan be accurately sampled during sample period 340. Following theexample outlined above in conjunction with FIG. 2, signal 325 representsa minimum value of Rcvr_Out_1, which becomes approximatelyVdd/2−Vdd/4=1/4 Vdd, when settled. Signal 330 represents an example ofan unsettled Rcvr_Out_1, which can occur during the presence of a fingeror other input object which also couples rapidly varying interferenceinto receiver amplifier 121-1. This rapidly varying interference causesreceiver amplifier 121-1 to slew and not achieve a settled state forRcvr_Out_1. An accurate sample of signal 330 cannot be taken duringsample period 340, because the signal is unsettled while the receiveramplifier 121-1 is operating in a non-linear, slewing state. Signal 335represents an example of a saturated Rcvr_Out_1, which can occur due tothe presence of very high interference being coupled into receiveramplifier 121-1 during positive drive. Any sample of signal 335 taken atsampling period 340 would be invalid because receiver amplifier 121-1 isoperating in non-linear saturated state. Location 345 representsRcvr_Out_1 during another reset.

Location 350 represents the beginning of negative drive of XMTR1. Duringnegative drive, the example Rcvr_Out_1 signals shown are essentially areflected version of the previously discussed signals. The onlydifference being the direction of the signals away from Vdd/2. Thus, forexample, instead of occurring at 0 volts as during positive drive,saturation occurs at Vdd during negative drive. Signal 355 represents anexample of baseline static present during the absence of a finger orother input object. Signal 360 indicates an example settled level ofRcvr_Out_1 when a finger or other input object is present and sensed.Signal 360 is settled enough that it can be accurately sampled duringsample period 375. Following the example outlined above in conjunctionwith FIG. 2, signal 360 represents a maximum value of Rcvr_Out_1, whichbecomes approximately Vdd/2−Vdd/4=3/4 Vdd, when settled. Signal 365represents an unsettled signal due to rapidly varying interference,coupled by a finger or other input object, which causes receiveramplifier 121-1 to slew rather than achieve a settled Rcvr_Out_1. Anaccurate sample of signal 365 cannot be taken during sample period 375,because the signal is unsettled due to receiver amplifier 121-1operating in a non-linear, slewing state. Signal 370 representssaturation caused by high interference being coupled into receiveramplifier 121 during negative drive. Any sample of signal 370 taken atsampling period 375 would be invalid because receiver amplifier 121-1 isoperating in non-linear saturated state.

As described herein, the Non-linearity_Out_1 signal from receiveramplifier 121-1 is set when receiver amplifier 121-1 is operating in anon-linear manner due to non-settling or due to saturation, both ofwhich are caused by excessive interference (e.g., excessively spiky,rapidly varying, or high amplitude noise) being coupled with the Rcvr_1input. It is appreciated, however, that off-the-shelf transconductanceamplifiers are not equipped with an output to indicate operation in anon-linear manner. Thus, FIG. 4 is provided as an expanded block diagramillustrating an embodiment of implementing a capacitive sensorinterference determining circuit with a receiver amplifier of acapacitive sensing device.

FIG. 4 is a block diagram of an example capacitive sensor interferencedetermining circuit 400 implemented with a receiver amplifier of acapacitive sensor device, according to an embodiment. It is appreciatedthat FIG. 4, also represents a more detailed expanded block diagram ofdetail 120 of FIGS. 1 and 2. Capacitive interference determining circuit400 comprises an amplifier 410, a current differential amplifier 430,and a current comparator 440. In one embodiment, an OR gate or otherhardware logic, firmware logic, or software implemented in hardware isincluded to provide an interference indicator 450.

Amplifier 410 includes a first input for receiving a signal (e.g.,Rcvr_1) from a receiver sensor channel of a capacitive sensor and asecond input for a reference voltage (e.g., Vref). In one embodiment,amplifier 410 is a transconductance amplifier. It is appreciated thatamplifier 410 acts as a receiver amplifier to amplify a signal, such asRcvr_1, into an output signal Rcvr_Out_1. Thus, amplifier 410 operatesin the manner previously described in conjunction with receiveramplifier 121-1. Circuit stage 415 is the interference determiningprocessor portion of FIG. 4. Circuit stage 415 is coupled with amplifier410 and detects non-linear operating conditions of amplifier 410, andthus determines the presence of too high of an interference level formeasurement sampling, which occur due to interference on Rcvr_1 beingcoupled into amplifier 410.

Current differential amplifier 430 is coupled with amplifier 410 in thesense that the values of Ipos and Ineg (from the current sources insideof amplifier 410) are coupled as inputs to current differentialamplifier 410. In one embodiment, the values of Ipos and Ineg aremirrored out of amplifier 410 by current mirror 420, as shown in FIG. 4.Current differential amplifier 410 outputs two differential currentsrepresenting differentials of currents, Ipos and Ineg, utilized inamplifier 410. As shown in FIG. 4, these outputs are represented asB1(Ipos−Ineg) and B2(Ineg−Ipos), where B1 and B2 represent constantsassociated with gain in current differential amplifier 430.

Current comparator 440 is a current comparator with hysteresis. Currentcomparator 440 is coupled with current differential amplifier 430 andreceives the two outputs from current differential amplifier 430 as ofthe inputs to current comparator 440. Current comparator 440 comparesthe two differential currents received from current differentialamplifier 430 (e.g., B1(Ipos−Ineg) and B2(Ineg−Ipos)) to two referencecurrents Iref_1 and Iref_2 to determine whether a non-linearitycondition exists in amplifier 410. In one embodiment, the non-linearitycomprises an occurrence of a saturation condition resulting in thesaturation of the output (Rcvr_Out_1) of amplifier 410. Such saturationis typically caused by very high amplitude interference being coupledinto the input amplifier 410 via Rcvr_1. In another embodiment, thenon-linearity comprises an occurrence of an insufficient settling of theoutput (Rcvr_Out_1) of amplifier 410. A lack of settling is typicallycaused by rapidly varying interference being coupled into the inputamplifier 410 via Rcvr_1, and causing amplifier 410 to slew.

The comparisons performed by current comparator 440 can reveal anon-linearity (due either to saturation or non-settling) because duringa settled, non-saturated state of amplifier 410, Ipos and Ineg will besubstantially equal to one another and thus have a difference equal tozero or near to zero. However, in a saturated state or in a non-settledstate, Ipos and Ineg will diverge from one another beyond somethreshold, which is measured by comparison to Iref_1 and Iref_2. Becauseof this, the values of Iref_1 and Iref_2 are set based on empiricallydetermined values such a that they represent a threshold differencevalue (e.g., B1(Ipos−Ineg) and B2(Ineg−Ipos)) beyond which Ipos and Inegare considered divergent enough from one another that amplifier 410 isoperating in a non-linear state and therefore samples contemporaneouslymeasured from of Rcvr_Out_1 will be invalid. The values of Iref_1 andIref_2 can be predetermined and preset in hardware of controller 105(e.g., unchangeable) or can be adjustable through an interface tocontroller 105 (e.g., programmable).

Current comparator 440 includes a first comparison output and a secondcomparison output. The first comparison output (Comp_1) outputs a resultof comparing a first of the two differential currents to a firstreference current of the two reference currents. For example, in oneembodiment, Comp_1 represents a comparison of B1(Ipos−Ineg) to Iref_1and is set when B1(Ipos−Ineg)>Iref_1. The second comparison output(Comp_2) outputs a result of comparing a second of the two differentialcurrents to a second reference current of the two reference currents.For example, in one embodiment, Comp_2 represents a comparison ofB2(Ineg−Ipos) to Iref_2, and is set when B2(Ineg−Ipos)>Iref_2. It isappreciated that amplifier 410 is considered to be operating in anon-linear fashion and thus the presence of too much interference foraccurate sampling is determined when either Comp_1 or Comp_2 is set. Inone embodiment, the values of Comp_1 and Comp_2 are communicativelycoupled to a sampling circuit or logic (not illustrated) and serve as anindicator that interference is detected due to non-linear operation ofamplifier 410. Thus, with reference to FIG. 3, if either Comp_1 orComp_2 is set during a sampling period (340, 375), then the samplingcircuit or logic is alerted that the sample should be consideredinvalid.

In one embodiment, interference indicator 450 is included to consolidatethe outputs of current comparator 440 (or a plurality of currentcomparators that are each associated with a different receiver amplifier121). As illustrated in FIG. 4, interference indicator 450 is a hardwareindicator in the form of an OR gate with the first comparison output(Comp_1) and the second comparison output (Comp_2) as inputs. The outputof interference indicator 450 is set when either of the first comparisonoutput (Comp_1) or the second comparison output (Comp_2) is true. It isappreciated that a hardware interference indicator is shown by way ofexample only, and that in other embodiments interference indicator 450is implemented in firmware (e.g., firmware of controller 105) orsoftware that is implemented on controller 105 or with some otherelectronic device.

Example Method of Determining Interference in a Capacitance SensorDevice

FIG. 5 is a flow diagram of a method of determining interference in acapacitance sensor device, according to an embodiment. Elements of flowdiagram 500 are described below, with reference to elements of FIG. 1-4.

At 510 of flow diagram 500, in one embodiment, the method transmits asignal on a transmitter sensor channel of a capacitive sensor. Withreference to FIG. 1, in one embodiment, this comprises transmitting adrive signal with XMTR1 on a row 111 of sensor array 110 of capacitivesensor device 100.

At 520 of flow diagram 500, in one embodiment, the method receives thetransmitted signal on a receiver sensor channel of the capacitivesensor, the receiver sensor channel being coupled with an amplifier.With reference to FIGS. 1, 2, and 4, in one embodiment, this comprisesreceiving the transmitted signal on a row 111 of sensor array 110. Thisis represented by the signal on Rcvr_1 being coupled as an input toreceiver amplifier 121-1 in FIGS. 1 and 2 and to amplifier 410 in FIG.4.

At 530 of flow diagram 500, in one embodiment, the method examinesbehavior of the amplifier for non-linearity to determine if a level ofinterference has been received by the receiver sensor channel inconjunction with receipt of the signal. With reference to FIG. 4, in oneembodiment, this comprises comparing the values of internal currentsources of the receiver amplifier to determine if a non-linearitycondition exists in the operation of the receiver amplifier due to thepresence of interference which causes either a saturation condition ofthe output or prevents settling of the output by a time when a sample istaken of the output. As illustrated in FIG. 4, in one embodiment, acurrent amplifier (e.g., current differential amplifier 430) is used toexamine a magnitude of an output current of the amplifier by determiningthe differences between the values (Ipos and Ineg) of internal currentsources of amplifier 410. In one embodiment, one or both of thesedifference magnitudes is compared, using current comparator 440, to oneor more reference currents (Iref_1, Iref_2) to detect the occurrence ofa non-linear operating condition of amplifier 410.

At 540 of flow diagram 500, in one embodiment, the method sets anindicator in response to an occurrence of a non-linearity in thereceiver amplifier (e.g., receiver amplifier 121-1 in FIGS. 1 and 2 andamplifier 410 in FIG. 4). This indicator can comprise a hardwareindicator, software indicator (implemented on hardware), or a firmwareindicator (implemented on hardware). A hardware interference indicator450 is illustrated as an OR gate in FIG. 4, with an output,Non-linearity_Out_1, that is set contemporaneously with when non-linearoperation of amplifier 410 is determined to be taking place.

Example Method of Fabricating an Interference Determining ProcessorSystem for a Capacitive Sensor Device

FIG. 6 is a flow diagram 600 of a method of fabricating an interferencedetermining processor system for a capacitive sensor device, accordingto an embodiment. Elements of flow diagram 600 are described below, withreference to elements of FIGS. 1, 2, and 4. It is appreciated that the“coupling” described in flow diagram 600 can comprise electricallycoupling and/or communicatively coupling items with one another.

At 610 of flow diagram 600, in one embodiment, the method couples anamplifier with a receiver sensor channel of the processor system. Withreference to FIGS. 1, 2, and 4, this comprises, for example, couplingreceiver amplifier 121-1 or amplifier 410 with a receive channel that iselectrically coupled with a column 112 of sensor array 110 in capacitivesensor device 100. As illustrated in FIGS. 1, 2, and 4 the amplifier(121-1, 410) includes a first input for receiving a signal (e.g.,Rcvr_1) from the receiver sensor channel and a second input for areference voltage (Vref). In one embodiment, the processor system isrepresented as circuit stage 415 of FIG. 4.

At 620 of flow diagram 600, in one embodiment, the method couples acurrent differential amplifier with an output of the amplifier. Asillustrated in FIG. 4, this can comprise mirroring out the currents Iposand Ineg from amplifier 410, using current mirror 420, and couplingthese mirrored currents as inputs to current differential amplifier 430.As illustrated in the embodiment of FIG. 4, amplifier 410 is atransconductance amplifier. The difference between these currents willcomprise the output current supplied through Rcvr_Out_1 of amplifier410. Current differential amplifier 420 outputs two differentialcurrents, B1(Ipos−Ineg) and B2(Ineg−Ipos), representing differentials ofthe currents, Ipos and Ineg, utilized in amplifier 410.

At 630 of flow diagram 600, in one embodiment, the method couples acurrent comparator with the differential amplifier. This is illustratedin FIG. 4, with current comparator 440 receiving the currentdifferential outputs, B1(Ipos−Ineg) and B2(Ineg−Ipos), from currentdifferential amplifier 430 as inputs to compare with reference currentsref and Iref_1 and Iref_2. The current comparator compares the twodifferential currents to two reference currents to determine whether anon-linearity exists in the receiver amplifier (amplifier 410). Forexample, as illustrated by the embodiment of FIG. 4, current comparator440 compares B1(Ipos−Ineg) with Iref_1 and compares B2(Ineg−Ipos) withIref_2 to determine whether a non-linearity exists in the receiveramplifier (e.g., amplifier 410). The non-linearity can represent anoccurrence of a saturation condition of the output of amplifier 410 oran occurrence of an insufficient settling of the output of amplifier410. If either of these conditions exists with the signal of Rcvr_Out_1,then amplifier 410 is operating in a non-linear range and a sample ofthe output measured during this non-linear operation will be invalid dueto the presence of interference causing either saturation of the outputor preventing sufficient settling of the output.

At 640 of flow diagram 600, in one embodiment, the method also couplesan interference indicator with the current comparator. As previouslydescribed, the interference indicator can be implemented with hardware,a combination of firmware and hardware, or a combination of software andhardware. For purposes of example, and not of limitation, hardwareinterference indicator 450 is illustrated in FIG. 4. The interferenceindicator (e.g., interference indicator 450) outputs an interferenceindication signal in response to comparisons of the reference currentsand the differential currents indicating presence of a non-linearoperating condition in amplifier 410. Thus for example, if either Comp_1or Comp_2 of current comparator 440 is set, then Non-linearity_Out_1 ofinterference indicator 450 will be set to indicate presence ofnon-linearity due to the presence of interference. However, when neitherComp_1 nor Comp_2 is set, then Non-linearity_Out_1 will not be set.

The foregoing descriptions of specific embodiments have been presentedfor purposes of illustration and description. They are not intended tobe exhaustive or to limit the presented technology to the precise formsdisclosed, and obviously many modifications and variations are possiblein light of the above teaching. The embodiments were chosen anddescribed in order to best explain the principles of the presentedtechnology and its practical application, to thereby enable othersskilled in the art to best utilize the presented technology and variousembodiments with various modifications as are suited to the particularuse contemplated. It is intended that the scope of the presenttechnology be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A processing system for a capacitive inputdevice, wherein said capacitive input device comprises a plurality ofsensor electrodes configured to detect input objects in a sensingregion, said processing system configured to: transmit a signal on atransmitter sensor channel of said capacitive input device; receive saidsignal on a receiver sensor channel of said capacitive input device,wherein said receiver sensor channel is coupled with an amplifier; anddetermine a level of interference received by said receiver sensorchannel in conjunction with receipt of said signal by using a currentamplifier to examine a magnitude of an output current of said amplifierand comparing said magnitude to a reference current to detect saidinterference.
 2. The processing system of claim 1, further configuredto: examine behavior of said amplifier for non-linearity to determine alevel of interference received by said receiver sensor channel inconjunction with receipt of said signal.
 3. The processing system ofclaim 1, wherein said capacitive input device further comprises adisplay overlapping said sensing region.
 4. The processing system ofclaim 1, wherein said processing system configured to determine a levelof interference received by said sensor channel in conjunction withreceipt of said signal by using a current amplifier to examine amagnitude of an output current of said amplifier and comparing saidmagnitude to a reference current to detect said interference furthercomprises said processing system being configured to: determine asaturation condition of said amplifier exists.
 5. The processing systemof claim 1, wherein said processing system configured to determine alevel of interference received by said sensor channel in conjunctionwith receipt of said signal by using a current amplifier to examine amagnitude of an output current of said amplifier and comparing saidmagnitude to a reference current to detect said interference furthercomprises said processing system being configured to: determineinsufficient settling of an output current of said amplifier exists. 6.A capacitive input device comprising: a sensing region; a plurality ofsensor electrodes configured to detect input objects in said sensingregion of said capacitive input device; and a processing system coupledwith said plurality of sensor electrodes and configured to: transmit asignal on a transmitter sensor channel of said capacitive input device;receive said signal on a receiver sensor channel of said capacitiveinput device, wherein said receiver sensor channel is coupled with anamplifier; and determine a level of interference received by saidreceiver sensor channel in conjunction with receipt of said signal byusing a current amplifier to examine a magnitude of an output current ofsaid amplifier and comparing said magnitude to a reference current todetect said interference.
 7. The capacitive input device of claim 6,further comprising: a display overlapping said sensing region.
 8. Thecapacitive input device of claim 6, wherein said processing system isfurther configured to: examine behavior of said amplifier fornon-linearity to determine a level of interference received by saidreceiver sensor channel in conjunction with receipt of said signal. 9.The capacitive input device of claim 6, wherein said processing systemis further configured to: determine a saturation condition of saidamplifier exists.
 10. The capacitive input device of claim 6, whereinsaid processing system is further configured to: determine insufficientsettling of an output current of said amplifier exists.